Automatic volume control circuit



April 10,-1945. F 1 FAL-NCQ y 2,373,180

AUTOMATIC VOLUME CONTROL CIRCUIT Filed Jan. 6, 1944 X Q l 'Patented Apr. 1 0, 1945A UNITED lsrrrus vrli'rnrn OFFICE AUTOMATIC VOLUME CONTROL CIRCUIT Frank J. Faltico, Minneapolis, Minn., assignor to The Maico Company, Incorporated,

apolis, Minn.

Minne- Application January 6, 1944, Serial No. 517,196 l 21 Claims.v '(Cl. 179-171) geously employed in electronic amplifiers operatnamely: anode andfcathode, are essential to this type of volume control and the signal level or amplitude at which rectification and consequent volume suppression will begin is customarily controlled by bleeding current from the amplifiers high voltage D. C. supply through a resistor or voltage divider and using part or all of the voltage drop across such resistor to bias the rectifier in a manner to prevent the flow of electrons from the rectier anode to cathode, except when the applied signal voltage rises to a suiiicient value to overcome the bias effect onl the rectifier and permit rectiiication, such biasing of the rectier elements being often referred to as the establishment of threshold or threshold voltage. In utilizing this type or character of volume control according to the hitherto acceptedY practice, either an additional tube was used in the amplier unit to provide the desired rectification or a special .dual-purpose tube having extra `elements useable for the purpose of rectiiication has been employed for biasing purposes. In either event,l however, the volume control system imposed an additional .load on the ampliiler"s high voltage D. C. source or required the use of an, additional D. C. high voltage supply source, and in cases where an extra tube was employed for A. V. C. action, an additional load was alsoplaced upon the ampliers low voltage iilament power supply. Oi course, in the case of portable amplifying units.v and particularly in the case of yvery lightland ultra-compact hearing aid ampliiier umts hich are customarily operated from self-contained small A and B dry cell batteries, the use of extra tubes or other partsfor A. V. C. action is highly undesirable asis also the imposing of any extra load upon the A'or B batteries to accomplish A. V. C. action; the latter because any increased load on the batteries essentially requires larger batteries for `equal life or results in shorter life for given size batteries. n

Primary objects of my present invention are. therefore, to provide improved methods and apparatus whereby volume suppression may be achieved in an electronic amplifier without placing any additional burden upon the amplier's source or sources of potential, without the use of extra source of potential, and without the use of extra tubes or special complex and often undesirable special purpose tubes. While these features are highly desirable in electronic ampliiiers generally, they are particularly desirable in connection with audio or voice amplii'lers of the compact and lightweight variety used in connection with personal hearing aids, in which, because all the elements of the hearing aid, including the amplier and its dry cell potential sources, are worn on the person ofthe user, long battery life and the reduction of bulk and weight to the greatest possible extent are primary objectives. Generally stated, the above and other important objects and advantages of my invention may be achieved in connection with a multi-stage electronic amplifier in the following manner, to wit: For the purpose of this example, it will be assumed that the amplierunit utilized has at least two stages employing an amplifying device in the nature of a vacuum tube in each of said'stages', and

is equipped in a stage preceding the' final stage .character and may be assumed to be coupled to an input circuit of a succeeding stage in conventional manner. This output circuit should include a source of D. C. potential maintaining the output anode-acting grid charged positively with Y respect to the cathode of thatv tube, so that the amplifier will function as such independently of the plate of said preceding stage tube. Now, for the purpose of volume control action, signal voltage from a succeeding stage may be fed back to the plate of said preceding stage tube, thereby causing the plate and cathode of said tube to function as the elements of a 'rectifier whenever peak positive values of signal voltage exceed the value of the positive D. C. potential impressed upon the grid of said preceding stage tube, and which is n ow serving as an output anode; it being understood that, because of the location of the output anode-acting grid of the preceding stage tube between the cathode and plate thereof, the positive D. C. potential impressed upon the said output anode-acting grid will prevent the ilow of electrons from the cathode to the plate of said tube except when the plate is given a positive charge exceeding that of the output anode-acting grid. From this, it will be clear that the positive voltage applied to the output anode-acting grid of the preceding stage tube serves as a threshold voltage which must be exceeded by positive peak values of signal voltagefed back to the plate from the succeeding stage to start rectification, vand that such relatively high signal voltage as is required to exceed the established threshold voltage will result only from positive feed-back signal voltage of high amplitude. With this arrangement, only A. C. signal voltage is preseni-l at the plate ofthe preceding stage tube when positive peak values of feed-back signal voltage are below the positive threshold potential impressed upon the output anode-acting grid of that tube. However, when the fed back signal voltage exceeds said positive threshold voltage, rectification takes place and there is available at the plate, in addition to an A. C. voltage, a pulsating negative voltage which may be utilized for automatic volume control action. In accordance with the preferred embodiment of the invention, this D. C. negative component, whenever itis present, is filtered to minimize pulsations and isthen impressed upon the control grid lof the preceding stage 2 as a bias trodes n, G1, G2, G1, and (P). The dectrodes I I are electrically energized iilaments which serve as directly heated cathodes; the electrodes (P) are the usual plates which are primarily intended to function as output anodes; and the electrodes G1, G2, and G3 are respectively the control grids, screen grids, and suppressor grids. 'Ihese grids G1, G1, and G3 are interposed between the electrodes II and (P) and are respectively spaced progressively further from the electrode I I.

The amplifier is provided with a pair of input terminals or binding posts I2 and I3, through which the amplifier may be connected to a suitable source of signal voltage such, for example," as a microphone (not shown). The input circuit to tube V1 is conventional and comprises the leadV parallel circuits comprising leads I6,`I'I, I8, and

I9. In accordance with the present example, the positive side of the battery (A) is connected to ground through a lead 20. As previously indicated, a grid of V1 that is closer to the plate. (P) than is the control grid G1 is connected in the manner of a plate to act as an output anode. In a tube of the character described, the screen grid G2 thereof is selected for this purpose, and, as shown, is connected in a suitable output circuit comprising alead 2|, a load resister 22, and a lead 23 .connecting to the positive side of a suitable source of D. C. potential, such as a B battery (B) having its negative side connected to ground to reduce the FVamplification factor or currentpassing capacity of that tube to thereby produce the desired volume suppression. However, it should be understood that it is within the spirit of the invention to impress this negative D. C.

component as va'bias on the control grid of any one or more of the amplifiers tubes.

The above noted and other highly important objects and advantages of the invention and the operation of a preferred embodiment of the invention will be made further apparent from the .following specification, claims, and appended Fig. is a. diagrammatic illustration of the giirect currentoomponent of the wave form of The amplifier of Fig. 1 is a two stage audio or voice frequency amplifier employing a multiple grid electronic amplifier in the nature of a vacuum tube in each of its successive stages, the'tube of the first stage being indicated by V1 and the tube of the second stage being indicated by V1. The tube V1 is a conventional high gain pentode radio tube, preferably of the variable mu type, andthe tube V2 is a conventional high power output type of pentode radio tube. The tubes V1 and V2 each comprise an envelope Il) and electhrough a lead 24. The input circuit to the control grid G1 of V2 is conventional and comprises a lead 25 having interposed therein a coupling condenser C2. Control grid G1 of V1 is biased negatively with respect to the cathode-acting Illament thereof through a groundlead 26, a dry cell or cells B1, and a resister 21. The screen grid G'1 of tube V2 is connected in conventional manner to the positive side of the B voltage supply through av lead 28 and series resistance 29. A by pass condenser C5 is preferably connected from G11 to ground, as shown.

The output anode or plate (P) of V2 is connected in a conventional output circuit comprising a lead 30 and a plate-loading choke 3I, one side of which is tied to the positive side of the B voltage supply l(B). piier is provided with a pair of output binding posts or terminals 32 and 33, the former being connected to ground by a lead 34, and the latter being tied to the plate (P) through a, lead 35, a condenser C3, a lead 36, and part of lead 3l. These output terminals 32 and 33 adapt the ampliiier to be connected to the input of any device or apparatus to be energized by signal voltage developed in the amplifier, such, for example, as

' an earphone 31 0r the like.

Now, it will be seen that, if the contra grid G1 of tube V1 is properly biased, the tube V1 will function as a triode amplifier and the complete The output side of this am-` bsck circuit' is established from the output of Vl and, as shown, this'feed-back circuit comconnected as the elements of a diode rectier inshunt across the resistor 4I by a circuit compris- Ving a lead 43, and leads I9 and 20. The resistor 4I ispreferably of the type having a sliding or adjustable tap 43, such as a conventional potentiometer, and this tap 43 is connected in a filter and time delaying circuit comprising a resistor 44', a. lead 45, a by-pass condenser C5, and a ground lead 46.

In order to obtain a normal zero bias on the grid G1y of tube V1, thesaid grid is connected to ground (B negative) through lead I5, a resistor 41, resistor 44, part of resistor 4I, and ground lead 42.

Preparatory to describing the operationin detail, attentionds directed to the fact that the relatively high positive D. C. voltage impressed upon the grid G2 of tube V1 serves as a threshold voltage which must be exceeded by positive voltage impressed upon the plate (P) of said tube before an electronic path can be established between the cathode-acting lament II and plate (P) of said tube. In other words, the high positive D. C. threshold voltage normally impressed upon the output anode-acting grid G11 'of V1 by battery (B) attracts the flow of electrons from the cathode-acting filament II of said tube away from the plate (P) of said tube and, therefore, prevents the flow of positive current from plate to illament of V1, except when the positive volt-v age impressed upon the plate of V1 exceeds the .positive potential impressed upon gridG2 of said tube. Now, therefore,'since the D. C. blocking condenser C1 is incorporated between leads 38 and 4U, it will be apparent thatI the only potential ever impressed upon the plate (P) of V1 is in the form of A. C. signal voltage and that such signal voltage will produce a current ow through the rectifier-acting elements,(P) land II of V1 when, but only when, the peak positive valuesof signal voltage impressed upon (P) of V1 exceed the positive D. C. potential impressedupon G2 of V1.

Operation Now, bearing in mind the facts just above recited and assuming that'a suitable source of signal voltage, such as a microphone, preamplifer, phonograph pick-up, vradio receiving set, or the like, is connected across the input terminals I2 and I3, it will be clear that the electrodes Il, G1, and G1'- of tube V1' will function as a triode amplier and that the amplier, as .an entirety, will function normally, except for autoas'raiso f l 3' value of the positive potential impressed upon G2 of V1 is indicated by the wave form line in Fig. 2, and the magnitude of the positive D. C.

44, condenser C5 and lead 46.

voltage (threshold voltage) impressed upon the plate G2 of V1 is indicated by the space between horizontal lines marked 0 and plus mark respectively in Fig. 2. Such A. C. signal voltage is, of course, passed to ground in its entirety, going in part through lead 42 and in part through resistor However, when the peak positive values of A. C. signal voltage fed back to the plate (P) of V1 exceed the positive D. C. threshold voltage applied to grid G2 of V1, such positive peak values of signal voltage as do exceed the said positive D. C. threshold voltage on grid G2 of V1 will be by-passed from plate (P) to lament I I of V1 and from thence to ground (B negative) through leads I9 and 20. The wave form of the voltage developed across the resistor 4I under these conditions is indicated' by the unbroken wave form line of Fig. 3. wherein the horizontal lines marked 0 and plus mark respectively correspond exactly to those of Fig. 2. Referring further to Fig. 3, it will be seen that the positive peak values of signal voltage that exceed the threshold voltage and are, therefore, by-passed through the circuit comprising the rectier elements (P) and II of V1, are indicated by the dotted Wave form line. The Wave form of Fig. 3 is representative of an A. C. voltage with a negative D. C. component! and this is quite clearly indicated by Figs. 4 and 5, the former of 40 by the unbroken wave form line of Fig. 4, will :be passed in its entirety to ground, going in part through lead 42 and in part through resistor 44, condenser C5,- and lead 4l; and a selected percentage of negative D. C. potential appearing across resistor 4I (the percentage depending upon the position of tap 43) will be by-passed through resistor 44 and impressed upon G1 of V1 through the grid resistor 41 and lead I5. .While this negamatic volume control action, independently of the plate (P) of V1. During the entire operation of this ampliiier, a c. signal vonage' develqped in the output circuit of the succeeding stage tube W is fed back to and impressed upon theplate (P) of the preceding stage tube V1, 'but so long as the peak positive values of such fed back signal voltage are below the'value of the positive D. C.v

potential impressed on the grid G2 of tube V1, no rectier action will take place between (P) and II of V1, and only pure A. C. voltage will tive D. C. voltage available for volume control biasing is initially of the pulsating variety, it should be understood that the filter network comprising resistor 44 and condenser C5 ilattens out such -pulsations to the extent that a steady negative D. C. potential is applied as a volume control bias to the grid G1 of V1. l

With this system of volume control, the degree of volume suppression obtained can readily be varied by merely shifting the tap 43 of resistor 4I so as to make more or less of the negative D. C. component availa'ble for volume control purposes; and, of course greater volume control action may be readily obtained by adding one or more stages between V1 and V2 or by increasing the amplification factor of V2.

In defining and describing the present invention, the term grid has been used in a broad and liberal sense to designate, without regard to between two other electrodes, to control the 'flow of electrons between said other electrodes responsive lto changes in applied potential; the lterm plate has been used in a broad and liberal sense to designate," withoutregard to its physical char- Aa series condenser.

acter, any electrode of an electronic amplifier to which electrons fiow from a more negatively charged cathode or emitter under control of at least two intervening grids";` and the term cathode is used in a broad and liberal sense to designate, without regard to its physical character, an electron emitter or source of electronic discharge of an electronic amplifier, without regard to wlteth'er or not it be heated or whether it be heated directly or indirectly.

While the tube V1 has been described as being of the type having a suppressor grid tied to its filament or cathode, it should be understood that the invention is not dependent upon the presence of such a suppressor grid for its successful operation. However, the suppressor grid, whenemployed as in V1, has been found to materially improve the over-all results of the system described by acting' as a capacity shield between positively charged grid G2 and plate (P), thereby preventing or largely eliminating capacitive coupling between G2 and (P). Hence, it will be seen that, whereas, the suppressor grid G3 of tube V2 functions in the normal accepted manner, the

` suppressor grid G3 of tube Vl contributes an entirely different but, nevertheless, important. function which, as indicated, is the reduction to a minimum of the capacity between grid G2 and plate (P).

What I claim is:

1. In a multiple stage electronic amplifier system in which one stage preceding the final stage employs an electronic amplifier having, in addition to the usual cathode, plate, and control grid, another grid which when electrically charged will modify the flow of electrons from said cathode to said plate, the said other grid of said preceding stage amplifier being connected as an anode in an output circuit that is connected to the input circuit of the succeeding stage in the manner usually assigned to a plate and having impressed thereon a positive D. C. potential of pre-determined value.V whereby the said electronic amplifier of the preceding stage and the amplifier system as an entirety will function normally, except for automatic volume control action, independently of the plate of said preceding stage amplifier, means feeding back' signal voltage from the output of a succeeding stage to the plate of said electronic amplifier of the preceding stage, whereby rectification will take place between the plate and cathode of said orecedinglstage amplifier when, but only when, peak values of positive signal voltage exceed the positive D. C. potential impressed upon the said other grid of the preceding stage amplifier, and means for applying a pre-determined percentage of the negative D. C. voltage available at the said plate as a result of rectification to the control grid of an electronic amplifier of the said Asystem to serve as a volume suppressing bias.

2. The structure defined vin claim 1 in which the said signal voltage feed back means to the plate of thepreceding stage amplifier includes 3. The structure ldefined in claim 1 in which the said signal voltage feed-back means to the plate of the preceding stage amplifier inch'des a serially connected condenser and resistance.

4. The structure definedin Aclaim 1 in which the said means for applying a negative D. C. potential as a volume suppressing bias includes a suitable filter network for filtering out the pulsations before application to the control grid.

5. The structure defined in claim 1 in which the said means for applying a negative D. C. potential as a volume suppressing bias includes a resistance capacity network shunting out A. C. signal voltage and flattening out the pulsations in the negative D. C. biasing voltage.

6. In a multiple stage electronic amplifier system in which one stage preceding the final stage employs an electronic amplifier having, in addition to the usual cathode, plate, and control grid, another grid which when electrically charged will modify the fiow of electrons from said cathode to said plate, the said other grid of said preceding stage amplifier being connected as an anode in an output circuit that is connected to the input circuit of the succeeding stage in the manner usually assigned to a plate and having impressed thereon a positive D. C. potential of pre-determined value, whereby the said electronic amplifier of the preceding stage and the amplifier system as an entirety will function normally, except for automaticl volume control action, independently of the plate of said preceding stage amplifier, a signal voltage feed-back circuit f1 om the output of a succeeding stage and which involves a direct current blocking condenser and a resistance, an electrical connection between the plate of said preceding stage electronic amplifier and a point in said feed-back circuit intermediate said condenser and said resistance and which establishesY a rectifier :hunt circuit around said resistance comprising the plate and cathode of said preceding stage electronic amplifier, the said positive D. C. potential impressed upon the said other grid of the preceding stage electronic amplifier serving as a threshold voltage tending to repel electronic ficw from cathode to plate of said preceding stage amplifier' except when positive peak values of signal voltage impressed upon the plate of said preceding stage amplifier exceed the said threshold voltage impressed upon the said othe;` grid of said amplifier, whereby thc plate and cathode of said preceding stage amplifier will function as the elements of a rectifier and an A. C. voltage with a D. C. component will be de- Vcloped across said resistance whenever positlvcv peak values of signal voltage fed back to the plate of sad preceding stage amplifier exceed the th; eshold voltage impressed upcn the said other grid of the last said amplifier, a shunt circuit around a pre-determined percentage of said resistance and which includes an A. C. voltage bypassing condenser, and an electrical connection frcm the control grid of'the said preceding stage electronic amplifier to a point in the last said shunt circuit before said condenser, whereby the negative component developed across said resistance under conditions before described will be impressed upon the control grid of the preceding'stage electronic amplifier as a volume suppressing bias.

7. The structure defined in claim 6 in which the said signal voltage feed-back circuit further includes a resistance in series with the said condenser and located between the succeeding stage and the plate of the preceding stage.

8. The structure defined in claim 6 in which the last said shunt circuit includes a lresistance located between the before mentioned resistance of the feed-back circuit and the condenser in the last said shunt-circuit.

9. Thestructure defined in claim 6 in which the electrical connection between the last said stage amplifier includes a grid resistance.

'10'. The structure defined in claim 1 in which grid that is closer to the plate than either of the before mentioned grids.

11. In a multiple stage"vacuum tube amplifier employing a multiple grid tube in a stage preceding the final stage thereof, said preceding stage involving anoutput circuit wherein a grid of said mutliple grid'tube other than the control grid is connected 'as signal vputput anode, said output circuit including a source of D. C potential impressing a positive voltage of predetermined value upon the last said grid, whereby said multiple grid vtube will function as an electronic coupling device 'and the amplifier will function as such independently of the plate of said tube,

' and an automatic volume control for said amplifier comprising a signal voltage feed-back circuit energized from the signal voltage output circuit of a stage succeeding the said multiple grid tube and which circuit includes the plate and cathode of said multiple grid tube of the preceding stage, whereby the plate and cathode of said preceding stage multiple grid tube will function as the elements of a rectiflfler when, but only when,` peak values ,of positive signal voltage fed back from the succeeding stage exceed the positive D. C. potential applied to the anode-acting grid of the last said tube, and a strictly D. C. coupling betweenthe rectifier-acting .plate of said multiple grid tube and the control grid on tube of the amplier, whereby negative D. C.

2,378,180 5 circuit' and control grid of the preceding ceeding stage, whereby the said screen grid tube of said preceding stage will function as an am plier independently of its plate, feeding backsignal voltage from the output circuit of a succeeding tube to the plate of the said screen grid tube of the preceding stage so that the said plate i and the cathode of said screen grid tube will function as a diode rectifier when, but only when, the peak positive values of feed-back signal voltage from the succeeding stage exceed the value of the D. C. potential applied to the screen grid serving as an output anode insaid screen grid tube, and impressing upon the control grid of a tube of the multiple stage amplifier a pre-determined percentage of the negative D, C. potential appearing at the diode-acting plate of said screen grid tube while isolating said control grid from the A. C. component at the said plate.

14. The structure defined' in `claim l in which the electronic amplifier of the preceding stage includes a grid that is closer to the plate than either of the aforesaidv grids and is tied to the cathode of said amplifier.

15. In a multiple stage electronic amplifier system in which one stage preceding the final stage employs an electronic amplifier having, in addition to the usual cathode, plate, and control grid,

another grid that is electrically closer to the potential appearing at the rectifier-acting plate of the multiple grid' tube of the preceding stage will be impressed, as a volume controlbias, upon the control grid of said other tube.

l2. A method of providing automatic volume control in a multiple stage vacuumhtube amplifier employing a multiple grid tube in a stage preced-v ing the final stage which comprises connecting a grid of said multiple grid tube other than the control grid thereof to operate as anlanode in a voltage output circuit which impresses upon the said anode-acting grid a positive D. C. potential of pre-determinedvalue, whereby said multiple grid tube of the preceding stage will operate as an electronic coupling device independently of its plate, feeding back signal voltage fromfthe succeeding stage to the plate of said multiple grid d l tube of the preceding stage so that the said plate and the cathode of said multiple grid tube will function as the elements of a rectifier when, but only when, the 'peak positive values of feed-back signal voltage from the succeeding stage exceed the value of positive D. C. potential applied to the anode-acting grid of the said multiple grid preceding'the nal stage which comprises con` necting the screen grid of the screengrid tube of said preceding stage to function as an anode in an output circuit for Said tube and coupling such output circuit to the input circuit of a sucplate than is said control grid, the said other grid of said preceding stage amplifier being connecting as an anode in an output circuit that is connected to the input rcircuit of the succeeding i stage in the manner usually assigned to a plate and having impressed `thereon a positive D. C. potential of pre-determined value, whereby the said electronic amplifier of the preceding stage and the amplifier system as an entirety will function normally, except for automatic 'volume control action, independently of the plate of said preceding stage amplifier, means feeding back signal.

voltage from the output of a 'succeeding stage to the plate of said electronic amplifier of the precedingstage, whereby rectification will take place between the plate and cathode of said preceding stage amplifier when, but only when, peak y values of positive signal voltage exceed the positive`D. C. potential impressed upon the said other grid of the preceding stage amplifier, and means for applying a pre-determined percentage of the negative D. C. voltage available at the said plate as a result of rectification 'to the control grid of an electronic amplifier of the said system to serve as a volume suppressing bias.

16. In a multiple vstage electronic amplifier sys-v tem in which one stage preceding the final stage employs an electronic amplifier having, in addition to the usual cathode, .plate and. control grid, another grid that is closer to the plate thanfis said control grid, the said other grid of said pre# ceding stage amplifier being connected as ran anode in an output circuit that is connected to the input circuit of the succeeding stage in the manner usually assigned to a plate and having impressed thereon a positive D. C. potential of pre-determined value, whereby the said electronic amplifier of the preceding stage and the amplifier system as an entirety will function normally, except for automatic volume control action, independently of. the plate of said preceding stage amplifier, a signal voltage feed-back circuit from the output of said succeeding stage'and which involves a direct current blocking condenser followed by a resistance, an electrical connection be- Ytween the plate of said preceding stage electronic amplifier-,and a point in said feed-back circuit' intermediate said 'condenser and said resistance and which establishes a rectifier shunt circuit around said resistance comprising the plate and cathode of said preceding stage electronic ampliiler, the said positive D. C. potential impressed upon the said other grid of the precedingstage electronic amplifier serving as a threshold voltage tending to repel electron ilow from cathode to plate of saidpreceding stage amplifier except when positive peak values of signal voltage impressed upon the plate of said preceding stage amplifier exceed the said threshold voltage impressed upon the said 'other grid of said amplifier, whereby the plate and cathode of said preceding stage amplifier will function as the elements of `a rectifier and an A. C. voltage with a D. C. component will be developed across said resistance whenever positive peak values of signal voltage fed back to the' plate of said preceding stage amplifier exceed 'the threshold voltage impressed upon the said other grid of the last said amplier, a shunt circuit around a pre-determined percentage of said resistance and which includes an A. C.

.ing the n al stage thereof,.said preceding stage involving an output circuit wherein a grid of said multiple grid tube thereof that is electricallyl stage will operate as an electronic coupling device independently of its plate, feeding back signal voltage from the succeeding stage to the plate of said multiple grid tube oi.' the preceding stage so said multiple grid tube of the preceding stage, e

and impressing upon the control grid of said mul# tiple grid tube of the preceding stage a pre-determined percentage of the negative' D. C. potential appearing at the plate of said multiple grid tube while isolating said control grid from the A. C. component at said plate.

19. A method ofproviding automatic volume control in a multiple stage vacuum tube amplifier employing a, multiple grid tube in a stage preceding the final stagewhich comprises connecting a grid of said multiple grid tubeother than the control grid thereof to operate as an anode in a voltage output circuit which impresses upon the said anode-acting grid' a positive D. C. potential of pre-determined value, whereby said multiple grid tube of the preced- .ing stage will operate as an electronic coupling device independently of its plate, feeding back signal lvoltage from the succeeding stage to the plate of said multiple grid tube of the precedcloser to the plate of said tube than is its control sgrid is connected as signal output anode, said output circuit including a source of D. C. potential impressing a positive voltage of pre-determined value upon the last said grid, whereby said multiple grid tube will function as an electronic coupling device and the amplifier will function as such independently of theplate of said tube, and an automatic VVolume control for said amplifier comprising a signal voltage feed-back circuit energized v.from the signal voltage output circuit of a stage succeeding the said multiple grid tube'and which circuitincludes the plate and cathode of said multiple grid tubeof the preceding stage, whereby the plate and cathode of said preceding stage multiple grid tube will 'functionas the elements of a rectifier when, butonly when, peak values of positive signalvoltage fed back from the succeeding stage exceed the positive D. C. potential applied to the anode-acting grid of the last said tube, and a strictly D. C. coupling between the rectifier-:acting plate of said multiple grid tube and the control grid of said tubel where'- by a negative D. C. potential appearing at the rectifier-acting plate of the multiple grid tube of the preceding stage will be impressed, as a volume :o'ritrol bias, upon the control grid of the last said 18. A method of providing automatic volume control in a multiple stage vacuum tube amplifier employing a multiple grid tube in a stage preceding stage so that the said plateI and the cathode of said multiple gridtube will function as the elements of a rectifier when, but only when, the peak positive values of feed-back signal voltage from the succeeding stage exceed the value of positive D. C. potentialapplied to the anodeacting grid of the said'multiple grid tube of -the preceding stage, and impressing upon the control'grid of said multiple grid tube of the p receding stage a pre-determined percentage of the negative D. C. potential appearing at the plate of said multiple grid tube while isolating said control grid from the A. C. component at said plate.

20. A method of accomplishing automatic volume control in a multiple stage vacuum tube amplifier employing a screen grid tube in a stage preceding the final stage which comprises connecting the screen grid of the screen grid tube -of said preceding stage to function as an anode in an output circuit for said tube and coupling such output circuit to the input circuit of a succeeding stage; whereby the said screen grid tube of said preceding stage will function as an ampliiier independently of its plate, fe'eding lback signal voltage from the output circuit of a succeeding tube to the plate of the said screen grid 4 tube of the preceding stage so that the-said plate ing thefinal stage which'comprises connecting a I and the cathode of saidA screen grid tube will function as a diode rectiiier when, but only when, the peak positive values of feed-back signal voltage from the succeeding stage exceed theV value-of the D. C. potential applied to the screen Y isolating said control grid from the A. C. coi'n-A ponent at the said plate.

2l. In a multiplev lstage electronic ampliiier v system in which one stage preceding the final stage employs an. electronic ampliiier having, in

addition to the usual cathode, plate, and control grid, another grid that is electrically closer to the plate than is said control grid, the said other grid of said preceding stage amplifier being connected as an anode in an output circuit that is connected to the input circuit of the succeeding stage in the manner usually assigned to amplifier of-the precedingstage, whereby rectication will take place between the plate and cathode. of said preceding stage ampli'er when, but only when, peak values of positive signal voltage'exceed the vpositive D. C. potential impressed uponthe said other grid of the preceding stage amplier, means for applying a predetermined percentage of the negative D. C.

voltage available at the said plate as a result of rectification to the control grid of an amplifier to serve as a volume suppressing bias, said electronic ampliers of the preceding and succeeding stages being in the nature of electronic vacuum tubes, and the tube of the preceding stage further including a suppressor grid that is closer to the 4plate than either of the before mentioned grids and is tied to the cathode of said tube.

FRANK J.`FALTICO. 

